Bone and pedicle screws are known from the state of the art. They serve, for example, for dorsal stabilization of the spine by means of transpedicular screwing. Pedicle screws are placed in the pedicles of respective adjacent vertebrae, whereupon an angularly stable connection is made between the respective axially superimposed pedicle screws and an axially extending rod or land. The pedicle screws and rods constitute a vertebrae stabilizing system.
A pedicle screw usually includes a screw shank extending in the axial direction and having a male thread to which a seating sleeve, the so called tulip, is connected on the screw head side. Said tulip is formed substantially U-shaped having opposite wall portions (sleeve flanks) and a gap formed therebetween and extending in the radial direction for the rod or land. The tulip is provided with a thread extending in the axial direction. The rod is inserted in the gap of the tulip and is fixed by means of a clamping screw also being referred to as set screw and being screwed with the thread.
When attaching a set screw to a pedicle screw for fixing the rod, deformation of the tulip may occur due to forces acting on the pedicle screw and especially on the tulip thereof, for example in the case of manipulation of vertebral bodies or in the case of pressing on the rod. Such deformations prevent or impede screwing of the set screw. Basically such deformations can be minimized or even prevented by an appropriate design, especially dimensioning, of the pedicle screw. However, those endeavors counteract a weight and size minimization as well as a reduction of material used. Basically two types of deformations may occur, namely those in which the thread of the tulip is deformed in the direction of the thread of the set screw and those of the opposite direction.
In the case of the first-mentioned deformations, those deformations preventing the set screw from being attached to the tulip thread are especially difficult, because the threads are no longer capable of engaging in each other. However, even in the case of minor deformation that still allows screwing the set screw may tilt relative to the axis of the tulip due to the deformation of the tulip. This may result in the fact that the thread chamfer, especially the one of the set screw, is damaged. A so called “cross threading” may occur, which means that the set screw is tilted with respect to the longitudinal axis of the screw shank and the male thread thereof so far that the start of the thread of the set screw, i.e. the run-in thread pitch or pitches, engage(s) in the wrong thread pitch of the female thread of the tulip, which may result in damage of the thread up to the uselessness of the set screw and/or the pedicle screw.
In order to reduce the impact of deformations that may disengage the threads of the tulip and the set screw, the use of appropriate thread tolerances as well as thread designs such as e.g. rectangular threads or counterdraft threads is known. Deformations of the tulip can also be reduced by the use of additional or appropriately designed instruments when screwing the set screw. For example, those instruments include a counterdraft which is engaged in the tulip and prevents or compensates and resets the deformation thereof in the radial direction. As especially for percutaneous application those instruments are intended to include, on the one hand, an as long slit as possible and, on the other hand, to have an as small outer diameter as possible, the mentioned stabilizing effect can be insufficient in a disadvantageous manner. Another drawback of such instruments is a visual impairment of the operating area. Also instruments may be used in which no counterdraft interacting with the tulip is possible or present.
From US 2014 0350605 A1 a pedicle screw system is known in which the set screw includes a chamfered head area which facilitates attaching in a female thread formed in the tulip. In addition, the female thread of the tulip includes a shortened first thread pitch so that the set screw can be inserted into the tulip and centered by means of the chamfered head area initially without any thread engagement and subsequently is rotated in said position with a thread engagement being brought about. Under certain circumstances the shortening of the thread pitch may constitute weakening of the thread in a detrimental manner.